A New Conception of Progress

With these questions in mind, we ask, why did the turbofan engine, once it emerged, so totally dominate commercial aviation? P&W’s JT8D low-bypass turbofan engine, which went into service in 1964, is still powering Douglas’s DC-9 and Boeing’s 727 and 737. High-bypass turbofans, like P&W’s JT9D, GE’s CF6, and Rolls-Royce’s RB.211, have powered virtually all wide-body aircraft since the late 1960s. (The high-bypass turbofans required once more the same sort of steps in core-engine specific-power and fan tip Mach number as the initial low-bypass engines had required, and hence they need a separate analysis.85) The economics of the turbofan engine helped shape commercial jet aviation and stabilize it technologically and economically, putting air travel within the reach of a much larger segment of the public than it would otherwise have been. In other words, the turbofan engine has dominated high-subsonic flight because these two were mutually constitutive and emerged in parallel. Until the latter became important, the former did not make sense, technically or economically.

The turbofan responded to the decline of the notion that commercial jet flight would continually progress along the axis of speed. Much of the technology underlying turbofans had developed for entirely different purposes. Compressors received a great deal of attention in both industry and government, but none of that effort specifically sought a turbofan; it focused on turbojets. Immediately after World War II it seemed obvious that the continued progress of commercial flight would move, like military flight, toward higher and higher speeds. The only real customer for aircraft gas turbine engines before the mid-1950s, especially in the U. S., was the military, and they rightly pursued speed, and hence supersonic flight, above all else. More than a decade of supersonic flight and jet engines were required before it became clear that commercial air travel would follow many pathways, but increasing speed would not be one of them. Until the late 1950s, engineers simply did not see high-subsonic flight as a technical, or commercial, frontier. (Military flight leveled in speed as well: the aircraft that holds the world speed record, even today, was developed in the years just before and after 1960.) High-subsonic jet flight emerged as a dominant category, and ever increasing speed declined in importance as a category of problems, simultaneously with GE’s and P&W’s efforts to develop turbofan engines. Progress scarcely came to an end at this point, however.

The turbofan episode illustrates a dramatic, yet subtle shifting, we might even say a turning, in the parameters of progress in the narrative of aviation. The ever – increasing advance of the raw, physical parameter of speed ended in the 1950s, as commercial aviation settled into the high-subsonic regime. As an indicator of this shift, consider the proliferation of performance parameters in this story: stage pressure-ratio, thrust-to-weight ratio, propulsion efficiency, specific fuel consumption, cost per passenger mile. Significant progress was made in each of these measures with the emergence of the turbofan and in the years since, but they are less visible to the naked eye, less viscerally physical than speed. (An engineer, though, might argue that thrust-to-weight ratio is as “natural” a physical parameter as Newtonian mass and velocity.) Today’s airliners, to the untrained eye, look much like the 707 of four decades ago; for comparison, consider that forty years before the 707 were the biplanes of World War I. Of course, appearance is misleading. Stability in configuration masks substantial changes in engines (as we have shown), as well as in wing design, materials, control systems, and numerous other systems. Hence, the progress narrative in commercial aviation remains, but embedded in newer, seemingly more artificial measures that define “success” for advanced technologies, measures that embody social assumptions in machinery. The significance of the turbofan engine, and its intricate history, derives from this turning: from outward parameters of physics to internal parameters of systems.

This turning is evident not only in the broad parameters which evaluate aircraft performance, but also in the fine-grained texture of engineering practice. Engineers, in the story we have told, relied heavily on non-dimensional parameters of performance. Vincenti has characterized such “dimensionless groups” as useful in relating the performance of models to the performance of working prototypes.86 We here identify two additional categories of such parameters. One, typified by the diffusion factor, provided independent variables for empirical correlations. Such parameters enable a great deal of complexity to be digested into a form that allows designers to interpolate and extrapolate reliably from past experience. Another category consisted of performance parameters like the pressure-ratio and efficiency of compressor and fan stages and the thrust-to-weight ratio and specific fuel consumption of engines. These parameters provide a generic way of characterizing the state of the art and advances in it; by decoupling issues of performance from issues of implementation, they allow such thoroughly different approaches to turbofan design as GE’s and P&W’s to be meaningfully compared. One way in which engineering research has contributed to the turbofan has been through identifying and honing parameters that enable past successes to be repeated and that open the way to processes of continuous improvement.